1. Field of the Invention
This invention is concerned with liquid crystal displays (LCDs) and particularly with transflective LCDs that achieve lower battery usage and higher contrast.
2. Description of the Related Art
Conventional transflective LCDs have a partially-reflective partially-transmissive mirror element (also known as a transflector) which reflects ambient light received from the viewing screen back through the LCD, and transmits backlight emission which is switched on when ambient lighting is low.
In conventional transflective LCDs, the utilization of light is inefficient because light is both reflected and transmitted at the same time, by the partially-reflective partially-transmissive mirror. Additionally, the transmission and reflection have fixed values. Since at any given time, the sum of the transmission and reflection of a mirror can not exceed 100%, the partially-reflective partially-transmissive mirror sacrifices efficiency by simultaneously operating as a reflector and a transmitter. Typically such mirrors have a 70-90% reflectance and a 10-30% transmission.
To compensate for such inefficiency, more battery power must be used to increase the backlight emission when operating in the transmissive mode in low ambient light. Likewise, contrast is lost when operating in the reflective mode, at high ambient light.
This invention provides a system and method which improve the efficiency of conventional transflective LCDs by replacing the partially-reflective partially-transmissive mirror with a tunable mirror. A tunable mirror is any device having controllable degrees of transmission and reflection.
The advantage of using a tunable mirror is that it can be switched between xe2x80x9creflectivexe2x80x9d and xe2x80x9ctransmissivexe2x80x9d modes to primarily reflect light when ambient lighting is high, and to primarily transmit light when ambient lighting is low and backlighting is needed. This saves battery life by reducing the amount of backlighting needed when operating the LCD in the transmissive mode, and increases contrast and brightness when operating in the reflective mode.
The electrochemical reversible mirror (REM) is a suitable type of tunable mirror for use with this invention. Additionally, tunable mirrors may be constructed from a plurality of optical elements, at least one of which has an electrically switchable optical property.
One suitable construction for the tunable mirror includes a cholesteric liquid crystal reflector with a quarter-wave (xcex/4) retarder. In this combination, the liquid crystal reflector is switchable between reflecting and transmitting states of operation to give the mirror its tunable characteristic.
Another suitable construction for the tunable mirror includes a reflective polarizer with a zero to half-wave (0-xcex/2) tunable liquid crystal retarder. Such retarder is and is switchable between xcex/2 and 0xcex states of operation to give the mirror its tunable characteristic.
A third suitable construction for the tunable mirror includes a cholesteric reflector with a negative quarter-wave to positive quarter-wave (+/xe2x88x92xcex/4) tunable liquid crystal retarder. Such retarder is switchable between +xcex/4 and xe2x88x92xcex/4 states of operation to give the mirror its tunable characteristic.
There are various possibilities for controlling the mirror and backlight. For example, mirror and backlight control systems may be employed which operate the mirror and backlight in tandem, such that when the backlight is switched on, the mirror is set to the transmissive state. Another possibility is to set the mirror and backlight controls automatically responsive to the level of ambient light.